Devices for providing controlled sizes of liquid metal drops are used to create both electrical connections as well as mechanical connections between two pieces. For example, it is known to form solder bumps on a surface of an integrated circuit, where the solder bumps are eventually contacted by wires or contact pads from a second body to provide electrical and mechanical connections to the integrated circuit. The liquid drops of solder may also be used to form conductive traces, other conductive patterns, and mechanical structures. Typically, these solder bumps are 50-200 microns high.
The solder is usually heated to be in a molten state and, after ejection, freezes when it comes in contact with the workpiece. In most cases, the now solid solder drops are again melted for fusing the solder to a wire, a contact pad, or any other part.
The liquid metal is typically a tin-lead solder, but other metals and alloys commonly used for solders, such as gold, silver, and copper alloys, are also used.
Typical drop generators consist of a small tank filled with a liquid material, such as heated solder, which is connected to a pump. The pump forces the liquid metal through an orifice so that a certain size drop of metal is ejected from the orifice. Droplets, as opposed to a steady stream of the liquid metal, are created by either capillary-wave instability of the stream or a controlled pulsing of the liquid stream. One type of pump used to eject drops from the orifice is a piezoelectric plate which mechanically applies pressure to the tank upon a voltage applied to the piezoelectric plate. A single drop would then be ejected for each voltage pulse applied to the piezoelectric transducer.
The output orifice is typically non-wettable and small enough such that the liquid metal in the tank does not leak from the orifice without pressure applied by the pump.
The known drop generators have deficiencies because of either a complex construction or fluctuations in the size of an emitted droplet. For example, the performance of the piezoelectric transducer is highly dependent upon temperature conditions, such that variations in temperatures will vary the droplet size. Also, piezoelectric transducers can operate only below their Curie Point, which, for present materials, limits their use to metals with melting points below about 300.degree. C. Construction of piezoelectric force generators typically requires an adhesive to couple the force generator to a deformable element. Long-term mechanical stability of this adhesive bond is a significant design hurdle for these devices. Temperature, cycle-life, bulk modulus stability, and other sensitivities add unreliability to such systems.
Additionally, some known drop generators require pressure regulators, filters, complex liquid metal plumbing, dissolved gas monitoring, and other requirements which add bulk, expense, and complexity to the liquid metal droplet delivery system.
What is needed is an improved drop generator for liquid metal, such as heated solder, which is simpler, less costly, and potentially smaller than existing designs and which ejects droplets having a more controllable size.